const array1 = [1, 2, 3, 4]; const reducer = (accumulator, currentValue) => accumulator + currentValue; // 1 + 2 + 3 + 4 console.log('array1.reduce(reducer) ====',array1.reduce(reducer)); // expected output: 10 // 5 + 1 + 2 + 3 + 4 console.log('array1.reduce(reducer,5) ====',array1.reduce(reducer, 5)); // expected output: 15